1) Field of the Invention
The present invention relates to an improved process for evacuating a thermally insulating jacket and in particular the jacket of a dewar or of another cryogenic device, like for instance dedicated pipings for the storage and/or the transport of cryogenic gases such as nitrogen, oxygen, hydrogen, helium, argon etc. or of other substances requiring the maintenance of a temperature different from room temperature, usually lower, the insulating features of said jackets being notoriously reached by means of vacuum and of an insulating material.
The new process is particularly advantageous, as it allows said jacket to be put in condition to work in a very short time.
2) The Prior Art
It is known, according to the common practice, to use for the purposes above dewars or pipings having a vacuum jacket in order to reach an adequate thermal insulation. As a further thermoinsulating measure, there is usually inserted in the jacket an insulating material like glass wool, expanded organic polymers (e.g. polyurethane and resins having various compositions) and, very frequently, the so called "multilayers". These latter are consisting of alternated sheets made from organic polymers (such as polyolefines), preferably showing a cross-linked structure, and of aluminized plastics, obtained for instance, as it is known, by coating a plastic film with aluminium by means of evaporation under vacuum.
It is also known that the vacuum, in said jacket, has the tendency to degrade along the time, because of the outgassing of the inner components and even of the walls (namely by emission of gaseous species like CO, N.sub.2, H.sub.2, H.sub.2 O, O.sub.2 etc.) and owing to possible "leaks" (namely penetration of an atmospheric gas). In order to maintain the vacuum, there is normally utilized a gas sorbing material, placed in the jacket and generally consisting of zeolites, molecular sieves, silica gel, activated carbon (charcoal) and other sorbing material having physical action. These substances have to be kept at a very low temperature, e.g. the temperature of liquid nitrogen or a lower one, in order to allow the sorption of the most part of such gaseous species. Moreover, the physical sorbing materials hereinabove do not effectively sorb hydrogen. Therefore it was required in the past for instance, to insert in the jacket palladium oxide, as disclosed by GB-A-921273. Pd oxide in fact converts hydrogen into water, according to the reaction: EQU PdO+H.sub.2 .fwdarw.Pd+H.sub.2 O (.varies.)
Water is then physically sorbed by the zeolites or by the other physical adsorbents present in the jacket.
A first drawback, coming from the use of these techniques, is due to the fact that the preparation of the insulating jacket of the dewars or of pipings above requires a very long time, because the activation of the physical adsorbents (zeolites and the like) requires an extended thermal treatment under pumping, which can even last several days.
The process length is mainly determined by the necessity of reducing as far as possible the residual amount of water contained in the physical adsorbents and in the insulating materials. A shorter time could be reached by drastically increasing the temperature of the thermal treatment of the system (e.g. beyond 150.degree.-200.degree. C.), but frequently this treatment can not be carried out because the involved materials are not consistent with such temperatures or because of practicality grounds or of grounds bound to the process economics.
A second drawback comes from the physical nature of the sorption, which makes the reaction reversible. Therefore, for instance, as a dewar is emptied and its inside is at room temperature, the sorbed gases are re-emitted thus giving rise to a certain pressure in the jacket. Consequently, during the filling of the inner vessel (of the dewar or of the piping), already returned to room temperature, there is observed a vigorous boiling, with a gas loss, because of the worsened insulation conditions. These are re-established after a certain time, once the zeolites have been cooled down to cryogenic temperature and have sorbed again the gases. Such a problem, known as "boil-off" or reboiling, is a serious one, especially in the case of liquefied gases like H.sub.2 and He, which are relatively expensive and endowed with a low heat of evaporation. It should also be underlined the fact that a few of the known materials used so far do react with the liquefied gas, with which they can occasionally get into contact, as soon as there is a leakage from the inside of the vessel, e.g. because of the formation of cracks. PdO in particular cannot be utilized with liquefied H.sub.2, because of possible explosions, in the case of breaking of the inner wall. The same occurs in the case of liquid O.sub.2, when activated carbon (charcoal) is used.
It was suggested by C. Boffito et al., in J. Vac. Sci. Technol. A5 (6), 3442 (1987), to use a gettering material, based on a Zr-V-Fe alloy disclosed in GB-A-2043691. This material is actually solving a few of the problems hereinabove, as it can chemically sorb the different gases, responsible for the degrading of the vacuum, and especially hydrogen.
The insulating material however gives rise, during the manufacturing process, to a heavy release of water, which can considerably jeopardize the effectiveness of the sorption, with respect to the other gaseous species, all along the life of the device.
It is therefore a first object of the present invention to provide a process allowing to shorten the time required for evacuating and making ready the insulating jacket of a dewar or of an other cryogenic device, such as for containing and/or transporting liquefied gases.
Another object of the present invention is to provide a process of the kind hereinabove, free from said "boil-off" problems, during the filling of the vessel with liquefied gases, onche the vessel has got empty and after it has come back to room temperature.
A further object of the present invention is to provide a process allowing the elimination of the hazard bound to some of the materials used for the maintenance of vacuum, in particular palladium oxide and activated carbon (charcoal). These materials can react in fact in an explosive way with hydrogen and oxygen, respectively, if either of the two gases are present in a liquefied form, inside the vessel to be insulated and if such a vessel is undergoing a breakage.
Still another object of the present invention is to provide a process for obtaining an insulated device (dewar or piping) granting an effective chemical pumping, with respect to the undesired gases, all along the life of the same device.